The present invention relates to sensors and in particular to micromechanical rotation rate sensors and methods for producing the same.
Micromechanical rotation rate sensors have been known for considerable period of time. They comprise one or several micromechanically structured seismic oscillating masses which are subjected to a controlled periodic movement (excitation movement) in a plane (excitation oscillation plane). These seismic oscillating masses are structured and fastened such that they or parts of them are suspended so as to be movable also in a plane at right angles to the excitation oscillation plane. This plane is determined as detection plane. These rotation rate sensors also comprise a detection unit which detects a deflection of the oscillating mass or of the oscillating mass or parts thereof in the detection plane.
The deflection in the detection plane either results from the Coriolis force FC on the moved oscillating masses in the case of linear oscillators, or it results from the angular momentum conservation in the case of rotational oscillators. The deflection depends on the rotational speed xcexa9, which is also referred to as rotation rate, and on the speed of the excitation movement (v or xcfx89). The deflection is directed perpendicular to the original excitation movement. Hence, the detection unit can convert the detection of the movement in the detection plane into a rotational speed or rotation rate of the sensor:
FC=2 mvxc3x97xcexa9
M="THgr"xcfx89xc3x97xcexa9
The peculiarity of sensors used for detecting rotation rates e.g. in comparison with accelerations sensors is that an excitation movement of the seismic oscillating masses is required, such an excitation movement being, of course, not necessary in the case of acceleration sensors. The rotation rate can be measured only indirectly via the speed or rotational speed of the additional excitation movement, which should, however, not have any further influence on the detection. This necessitates a large number of degrees of freedom of movement.
Various production technologies and their boundary conditions limit the possibilities of realizing the structures so that the arrangement must be adapted to the production technologies used. The production technology must therefore be compatible with the complex structure of the whole rotation rate sensor.
Various realization principles in combination with different production techniques are known in the prior art.
DE 195 00 800, for example, shows rotation rate sensors comprising two masses which oscillate linearly relative to one another. The masses are formed by structuring polysilicon that has been grown epitactically on a substrate.
DE 195 23 895 shows rotation rate sensors as rotational oscillators utilizing the angular momentum conservation. These rotation rate sensors are produced in a manner which is similar to that described in DE 195 00 800.
DE 195 28 961 shows rotation rate sensors in the form of a tuning fork, the two prongs being structured from different wafers an subsequently combined so as to obtain the sensor.
WO 98/15799 discloses rotation rate sensors with decoupled orthogonal primary and secondary oscillators which can be produced by means of micromechanical processes, micromechanical surfaces technologies or in a way similar to that described in DE 195 00 800.
The known production methods show a plurality of disadvantages. When surface micromechanical techniques are used for structuring on grown polysilicon the oscillating masses, the torsion bars and torsion springs and the suspensions, stresses will occur in the micromechanical element which are caused by the epitactically grown polysilicon. These stresses result in a change in the mechanical behaviour.
Furthermore, epitactically grown polysilicon can only be produced up to a certain height, whereby the sensor element will be limited to a certain height. It follows that sufficient freedom for determining an optimum aspect ratio of structures does not exist; this may have a disadvantageous effect on the dimensioning, the sensor resolution and the interference susceptibility of the micromechanical rotation rate sensor.
Epitactically grown polysilicon is additionally subjected to ageing processes, in particular when it is constantly acted upon by a mechanical load, whereby the properties of the micromechanical rotation rate sensor may deteriorate with time. This kind of micromechanical rotation rate sensor will age.
DE 195 28 961 discloses with regard to the production of rotation rate sensors in the form of a tuning fork that the oscillating masses of the rotation rate sensor are produced from a plurality of SOI wafers, which have been preprocessed by bulk-micromechanical techniques, these oscillating masses being then connected so as to form sensors. A substantial disadvantage of this method is that the prongs of the fork are not adjusted in a sufficiently precise manner relative to one another and that it is difficult and expensive to match the mechanical properties. A further disadvantage is to be seen in the use of bulk-micromechanical technologies for structuring the prongs, whereby the possibilities of dimensioning the structures are strongly limited.
DE 195 26 903 A1 discloses a rotation rate sensor consisting of a multilayer substrate having a lower silicon layer and an upper silicon layer. The two si!icon layers have provided between them an insulating sacrificial layer. The lower silicon layer is implemented as a silicon wafer having a silicon oxide layer, a silicon nitride layer or glass applied thereto as a sacrificial layer. The upper silicon layer is produced by deposition from a plasma or by bonding a further silicon wafer to the sacrificial layer. The upper silicon layer may comprise polycrystalline silicon material, monocrystalline silicon material or a mixture of poly- and monocrystalline silicon materials.
The technical publication by A. Benitez, et al, which is entitled xe2x80x9cBulk Silicon Microelectromechanical Devices Fabricated from Commercial Bonded and Etched-Back Silicon-on-Insulator Substratesxe2x80x9d, Sensors and Actuators, A 50 (1995), pp. 99-103, discloses so-called BESOI substrates. BESOI stands for xe2x80x9cbonded and etched-back silicon-on-insulatorxe2x80x9d. A BESOI substrate is produced as follows. A starting substrate consisting of silicon is provided with an intermediate silicon dioxide layer. This intermediate silicon dioxide layer has applied thereto a further silicon wafer by means of wafer bonding or by means of wafer fusion, whereupon one of the two silicon wafers is etched back to the desired thickness. The silicon dioxide layer provided between the two wafers can be used as a sacrificial layer so as to produce electrostatic micromotors, microturbines and electrostatically operated microrelays.
It is the object of the present invention to provide a concept for micromechanical rotation rate sensors that can be produced at a moderate price, this concept permitting in addition a great freedom of design.
This object is achieved by a micromechanical rotation rate sensor according to claim 1 and by a method for producing a micromechanical rotation rate sensor according to claim 14.
One advantage of the present invention resides in the fact that it provides micromechanical rotation rate sensors which consist of a suitable material and in the case of which a very great freedom of design exists, without any necessity of matching the structures.
The present invention is based on the finding that the concept of a one-part rotation rate sensor, which is produced from a single wafer with epitactically grown polysilicon or with an SOI structure, must be departed from so as to achieve an optimum freedom of design and an economy-priced producibility. Hence, a micromechanical rotation rate sensor according to the present invention comprises a substrate wafer arrangement, a structural wafer arrangement in which there are defined at least one seismic mass, the suspension of this seismic mass and at least one spring means for connecting the suspension to the seismic mass, and an insulating connecting layer which mechanically connects the substrate wafer arrangement to the structural wafer arrangement in such a way that the seismic mass can carry out an excitation oscillation and that the seismic mass or parts thereof can carry out a detection oscillation on the basis of a rotation rate relative to the substrate wafer arrangement.
It follows that the micromechanical rotation rate sensor is based on a wafer stack arrangement, whereby the wafer arrangement for the substrate and for the rotation rate sensor as well as the materials of the rotation rate sensor can be chosen completely independently of one another and whereby the wafers can, in addition, be partly preprocessed before they are connected. In contrast to SOI substrates or substrates with grown polysilicon, the thickness of the structural layer, i.e. of the springs, the seismic masses, etc., can be chosen absolutely freely, simply by choosing a wafer having the desired thickness and by glueing it to the substrate wafer arrangement by means of the connecting layer, which can consist of polymer or of some other organic material, or by connecting a structural wafer via the connecting layer to the substrate wafer, which is then adjusted to the desired height. The connecting layer has the additional advantage that it can be used as an etch stop layer for the structuring process and as a sacrificial layer for obtaining freestanding structures.
According to a preferred embodiment with capacitive detection of the movement of the seismic mass on the basis of the Coriolis force, the substrate wafer arrangement comprises a metallization which is structured so that it comprises at least the detection electrodes. The metallization can easily be applied to a semiconductor wafer prior to connecting the metallized wafer via the connecting layer to the structural wafer. The complexity of the arrangement can be reduced in this way, since it is not necessary to pay attention to the fact that all electrodes have to be located on top of the structure, as may e.g. the case with SOI rotation rate sensors.
The layer or wafer stack arrangement leads to an arrangement which is optimal for the use of suitable material, the structuring of the elements and the positioning, dimensioning and application of the structures as well as the use of suitable excitation and detection units. The wafer stack arrangement permits the greatest possible freedom with respect to the design of the rotation rate sensors and with respect to different detection units. The excitation oscillation can be excited by various methods, such as piezoelectric, electrostatic, electromagnetic, electrothermal, inductive or thermomechanical methods. The detection of the measuring effect can also be realized by various detection units, among which e.g. the piezoresistive, the capacitive, the inductive, the optical, the piezoelectric and the thermomechanical ones should be mentioned. In the case of rotation rates with different excitation and detection units, the excitation oscillation can be realized as linear oscillation, as rotational oscillation or as torsion. Also for the detection a linear deflection or oscillation, a rotational oscillation or a torsion can be utilized.